1. Field of the Invention
The present invention relates to a bias circuit mounted in a power amplifier module used for a transmitter of a wireless system for example, and a method of producing a semiconductor device for the bias circuit.
2. Description of the Related Art
In a power amplifier module of a wireless transmitter, in particular, a power amplifier module used for a transmitter for a mobile phone, the bias current of the field effect transistor (FET) serving as the amplification element is set to a predetermined value. This is an important factor in determining the efficiency of power addition, distortion characteristic, or other aspects of the performance of the power amplifier module.
To determine the operating voltage and operating current of an FET, a bias circuit is connected to the gate. The bias circuit divides the voltage supplied from a gate bias supply terminal by resistance and supplies it to the gate. When the threshold voltage of the FET varies, however, there is the disadvantage that the operating current will not become constant if dividing the voltage by a constant resistance.
Further, one practice is to adjust the bias to the gate after completing the FET, but trimming of the resistance becomes necessary for each transistor, so there are the disadvantages that the trimming time increases and the area of the bias circuit becomes large.
Japanese Unexamined Patent Publication (Kokai) No. 9-283710 discloses a bias circuit for overcoming the above disadvantages. It uses an element of the same structure as the channel of the FET as the resistance. If possible to fabricate a resistance linked with the threshold voltage in a semiconductor substrate in this way, it would be possible to reduce the size of the bias adjustment circuit and drastically reduce the time and labor of a bias adjustment.
Summarizing the problems to be solved by the invention, if the threshold voltage becomes positive, a sheet resistance of the channel becomes one of an order of MΩ to GΩ. Such a resistance cannot be used for a bias circuit. This is because when the voltage supplied to the gate electrode is 0V, the channel of an enhancement mode transistor is basically designed not to pass current.
In particular, there has been a strong demand for fabrication of an enhancement mode transistor, which makes the threshold voltage positive and reduces the off current when the gate voltage is 0V for the transistor used for a power amplifier for the purpose of reducing the power consumption.
Therefore, a bias circuit has been sought that is able to be used even when the threshold voltage is positive and is able to make the bias current constant without trimming.